Effect of bacterial dose on pneumonia induced by aerosol exposure of calves to bovine herpesvirus-1 and Pasteurella haemolytica.

Sixteen crossbred Hereford range calves were used to study the effects of Pasteurella haemolytica aerosols of various concentrations administered 4 days after they were exposed to a uniform aerosol of bovine herpesvirus-1 (BHV-1). All calves were given BHV-1 on day 0. On day 4, groups of 4 calves each were exposed to aerosols of P haemolytica generated from suspensions with the following concentrations: high (10(8)/ml)--group 1, moderate (10(5)/ml)--group 2, low (10(2)/ml)--group 3, or none (control)--group 4. All calves developed clinical signs of respiratory infectious bovine rhinotracheitis. Those in group 4 (controls) and group 3 (low bacterial concentration exposure) were near complete recovery by day 8. At necropsy, most calves in all groups had diphtheritic inflammation of the mucosal surfaces of the upper and lower respiratory tracts and lobular atelectasis of the lung parenchyma. Three of the group 2 calves (exposed to the moderate concentration of P haemolytica) and all group 1 calves became severely ill with fibrinous pneumonia; 3 in the latter group died by day 8. For the conditions of this experiment, the 50% lethal dose was 1 X 10(7.1) bacteria/ml of suspension from which the aerosol was generated and 1 X 10(5.1) P haemolytica inhaled/calf. The 50% effective dose for fibrinous pneumonia was 1 X 10(4.1) P haemolytica/ml of suspension and 1 X 10(2.1) bacteria inhaled/calf. The term "inhaled" was used to mean the number of organisms estimated to be taken into the respiratory tract at the level of the nostrils. The results indicated that there is a positive relationship between degree of exposure to P haemolytica and severity of pneumonia in this disease model.